diff-files: fix incorrect usage of an empty tree

In c26022e (diff: convert diff_addremove to struct object_id,
2017-05-30), the OID to use for intent-to-add files was inadvertently
changed from the empty blob to the empty tree.

Let's revert that.

To be able to do that, we just taught the regression test introduced in
feea694 (diff-files: treat "i-t-a" files as "not-in-index",
2020-06-20) to _not_ expect the raw diff to contain the hash of the
empty tree (we also had to fix the code to actually produce the expected
output, but for the sake of this here patch, that's beside the point).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

diff-lib.c

@@ -234,7 +234,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 			} else if (revs->diffopt.ita_invisible_in_index &&
 				   ce_intent_to_add(ce)) {
 				diff_addremove(&revs->diffopt, '+', ce->ce_mode,
-					       the_hash_algo->empty_tree, 0,
+					       the_hash_algo->empty_blob, 0,
 					       ce->name, 0);
 				continue;
 			}